Because we are not very intelligent.  What else can I say?  We are conditioned. We are conditioned to be unintelligent. The temptations of this world are always upon us. You see, if you continue an activity, you become habituated to that activity and you become callous to the reality of the consequences. Just like if you are afraid to do something at first, but then you do it and get away with it, and you do it again and again. Pretty soon you become so callous to the consequences, you become so habituated, that you have no fear. Fear or guilt toward sinful activities is due to the presence of the Lord in our heart giving us some direction. We find sometimes when a child begins to smoke or drink, at the beginning he’s afraid. What will happen? After he does it for some time or she does it for some time, they become callous to the voice of the Lord within their heart and callous to the voice of their parents warning them. They’ve conditioned themselves to those activities. So, it is because we have ignored the directions of the Lord within our hearts since time immemorial, that oftentimes we have no fear and no guilt of performing any abominable activity. We have created that consciousness upon ourselves. Spiritual life means rehabituating ourselves, reconditioning ourselves to be habituated to those activities which are glorious and purifying.

It is said in Bhagavad-Gita that what tastes like nectar in the beginning, tastes like poison in the end for a conditioned soul; and what tastes like poison in the beginning, tastes like nectar in the end. To give up the sensual pleasures of life seems like poison in the beginning. But when we learn to utilize our same senses in God’s service, it tastes like sweet eternal nectar in the end. To enjoy the superficial things in this world tastes like nectar in the beginning, but in the end it is all taken away and has a poisonous taste. Therefore, we have to recondition and rehabituate our consciousness through constant practice. Then, when we become a little reconditioned in our consciousness, we become afraid of abominable activities, and we no longer fear the sweetness and the beauty of pure surrender in love to the all-glorious, all-loving Personality of Godhead.